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I guess I should chime in with some modifications I made to this preamp this weekend.

I have a pretty nice little subwoofer with built in plate amp that I decided to try out with my existing setup. I've got a 4S universal with 12au7 running into a 3W SET amp (6AS7). My speakers are full range high efficiency (Fostex 206e), but I felt like it might be nice just to give a LITTLE bit more to the low end (50hz and down).

Chatted with Matt a bit, and came up with the attached schematic.I moved the potentiometer up front replacing the fixed 499K grid leak resistor with a 250K pot. This lowers the input impedance to 250K obviously, but it's not an issue with any of my sources.

Then I replaced the 250K pot (previously located on the output) with a fixed set of resistors equaling 250K. The fixed value on the output of the stage fix the output impedance to about 11K which is nice since I'm now also feeding a solid state subwoofer plate amp which I expect doesn't have an overly high input impedance (can't find the value online).

You can see that I used a 220K ohm and 27K ohm combination of resistors to keep that same 250K resistance on the output, but I've created a voltage divider in doing it this way so I can have a much lower output for the subwoofer amp. According to matt the subwoofer output configured like this is about -18.5dBv from the main preamp output and still maintains the ac load on the 4S.

My subwoofer plate amp has line level rca inputs, and also controls to set the low frequency limit (Set to 50Hz currently), it also has a level control, and a phase control.Initially I tried running a second output to the sub directly from the main output of the preamp, but even with the volume control all the way down on the sub amp the level was way too hot. Using the voltage divider instead of a single 250K on the output puts the preamp's sub output level at a much more useable amount.

Anyways, I'm quite happy with this little modification, and I like the little bump to bass that I get using the sub with my little SET amp.

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The fixed value on the output of the stage fix the output impedance to about 11K which is nice since I'm now also feeding a solid state subwoofer plate amp which I expect doesn't have an overly high input impedance (can't find the value online).

Actually, the output impedance of the main output is ≈11.5kΩ. The output impedance of the attenuated output is more like (11.5kΩ+220kΩ)||27kΩ ≈ 24.2kΩ. Still plenty low so it shouldn't be a problem. Most SS amps have a fairly large input impedance.

I tossed in an old 12BH7 (I didn't take note of the brand, sorry) in the 4S and had it between an older Panasonic tuner and receiver.

It definitely wasn't an ideal listening session as I was cleaning up my shop area and was using some power tools on the other side of a partition wall, but it sounded fine.

One thing I did notice was that the envelope was taller than the 12A_7 family of tubes. Can this have an effect on the sound either by having more space for the electrons to whiz about in the vacuum or by having more room for a bigger ... plate or something? I realize it's not an absolute bigger glass tube is better than smaller glass tube thing, but what are the potentials?

I looked up the pin out (9a) in the RC30 and it had a few other 12B_7 tubes listed, any chance that they will have a similar operation?

Well, one could try the 12BZ7 (looks close to a 12AX7) or the 12FV7 (looks close to a 12AU7). Basically so long as it's a 9A pinout, the triodes are identical, and the filament current is in not too high, you could try just about anything. The fact of the matter is that there are a lot of tubes which are very close to each other. So go forth, roll tubes, and report what you find.

As for the envelope, most of the little twins are the 6B outline. The 12BH7 is a T-6 1/2 as are the 12BZ7 and 12FV7. The bulb size actually has absolutely no effect on the sound what so ever (waiting for the fury of the internet to arrive over that comment). It is really more about the mechanics of fitting everything in the tube. You can make the electrodes longer to allow more plate dissipation but again, no effect on sound. However, one thing for which a larger bulb can be used is to lower inter-element capacitance. This can be critical in tubes intended for a specific function (like sync separation and amplification (e.g. the 12BZ7). On the 12BZ7 Sylvania data sheet, you'll find a specification for plate to plate capacitance. This is important in some higher frequency applications.

I am trying to get my first tube preamp build and like the simplicity of the 4S design for a newbie. I wanted to build the power supply using the 24V wall wart since it looks simpler than the tube based power supply in the later design update.

I had a few questions on the schematics (and thanks in advance for your patience in these newbie questions):

1. The schematic shows one channel with the volume pot (P1) at the output. For stereo, does this mean there are separate left/right volume control knobs for level control? Can you indicate how the volume control knob merges for both channels. I am a little confused since the teabox build shows only one volume knob2. For the power supply, what should be the rating for the bride rectifier diode? Can you share the model/part number for it?3. For the 12a*7 tube, I assume pin 9 (heater center tap) should be left floating?4. Since I am new to valves, I assume the left/right channels can be amplified in a single valve using the 1-2-3 and 6-7-8 pin pairs?5. I am also a little confused about the grounding - If the wall wart has a chassis ground, can (should) I connect the chassis ground to the transformer body and the rest of the grounds? Or will this create ground loops/hum?

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